Layered structures

ABSTRACT

A process of making a product which comprises at least two layers in contact with each other, each layer being of a wide-gap material and each layer differing from each other in at least one property, includes the steps of: (i) providing a substrate of a wide-band gap material having a surface and a region adjacent the surface having a particular characteristic, (ii) ion implanting the substrate through the surface to form a damaged layer below that surface, (iii) growing a layer of a wide-band gap material by chemical vapour deposition on at least a portion of the surface of the substrate through which ion implantation occurred, the material of the grown layer having a characteristic different to that of the region of the substrate adjacent the surface through which ion implantation occurred, and (iv) severing the substrate through the damaged layer. The wide-gap material is preferably diamond.

BACKGROUND OF THE INVENTION

This invention relates to layered structures, particularly layereddiamond structures.

For certain diamond applications, it is necessary to have two or morelayers of diamond which have different properties in atomic contact witheach other. One example is in electronics where a layered structures canbe used to make a device, for example as described in WO 01/18882 A1

One method of making a layered diamond structure is by using ionimplantation. Ions may be implanted into diamond to create an n-type orp-type semi-conducting layer on top of a layer with a differentproperty. This method has the disadvantage that damage to the diamondoccurs during ion implantation which can result in permanent degradationin important properties such as carrier lifetimes and mobilities.Further, the types and concentration of ions which can be successfullyimplanted in diamond are limited and the process often requires complexpost implantation annealing.

Although chemical vapour deposition (CVD) provides a method ofsynthesising epi-layers, i.e. an epitaxially grown layer, to a desiredthickness, the CVD process enables only a few very specific defects andimpurities to be incorporated into the layer. This is best illustratedwith the following example. A boron doped diamond layer can be grownonto a high purity single crystal substrate using a CVD process known inthe art. The substrate might be processed from a natural diamond ordiamond synthesised by CVD or high pressure high temperature (HPHT)methods. This will produce a two layer structure. Using conventionalterminology this diamond structure will have pi-properties (i.e.properties exhibited by a sharp p-type to intrinsic semiconductorinterface). Many of the typical two layer device structures that mightbe produced in this way require one of the layers to be very thin (<20μm). For instance a 10 μm thick boron doped diamond layer on a 500 μmthick high purity diamond layer. A structure of this form, where precisethicknesses and sharp interfaces are necessary, requires considerablecontrol of the synthesis process and/or careful mechanical processing ofthe structure following growth.

A CVD process is conducive to the synthesis of thin epi layers but hasthe disadvantage that only layers containing certain dopants can besynthesised. For example it is well known that HPHT synthesis provides amethod of incorporating nickel, cobalt and nitrogen into the diamond inhigh concentrations (>5 parts per million (ppm) carbon atoms) but todate this has not been possible using CVD methods. Thus, to produce adiamond structure that consists of a thin epi-layer (<20 μm) containingnickel and a thicker boron doped layer (>100 μm) it would be necessaryto take a suitably prepared substrate containing Ni with a thicknesstypically >200 μm (for ease of handling and processing) and thensynthesise, using a CVD method, an overlayer (>100 μm) which containsthe required boron concentration. Following growth, considerable carewould then be needed with mechanical processing to finish with astructure which consists of, for example, a 10 μm Ni doped layer and a100 μm B doped layer where the thickness tolerances are better thanabout 2 μm.

U.S. Pat. No. 5,587,210 describes a method of separating a CVD diamondlayer from a diamond substrate. The method includes the steps of ionimplanting a diamond substrate, thus creating a damaged layer ofnon-diamond carbon below the surface of the substrate through which ionimplantation occurred, growing diamond on the surface of the substratethrough which the ion implantation occurred, and electrochemicallyetching the diamond substrate to remove the damaged layer. The resultingproduct is a free standing CVD layer having a thin, i.e less than 1000nm, layer of diamond bonded to a surface thereof.

The CVD diamond layer which is grown on the diamond substrate is pureCVD diamond. There is no suggestion that the CVD diamond layer should bedoped or otherwise treated to change its electronic or other properties.

This US patent also suggests that the diamond substrate can be doped byion implantation with suitable atoms to create n-type and p-typesemi-conductors. When ion implanting a diamond substrate to create suchsemi-conductor properties, the implanted region will vary considerablyin its dopant content. The region of highest and most uniform dopantconcentration will lie below the surface through which ion implantationoccurred. The region adjacent the surface through which the ionimplantation occurred will contain little or no dopant and ofnon-uniform concentration. Thus, on either side of the interface betweenthe substrate and the CVD diamond layer the material will be essentiallypure diamond.

Furthermore, ion implantation doping is always associated with latticedamage due to the ion implantation, which substantially reduces thebenefit obtained from the dopant in that it adversely modifies theelectronic properties of the doped layer.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process ofmaking a product which comprises at least two layers in contact witheach other, each layer being of a wide-band gap material and each layerdiffering from the other layer in at least one property, including thesteps of:

-   -   (i) providing a substrate of a wide-band gap material having a        surface and a region adjacent the surface having a particular        characteristic,    -   (ii) ion implanting the substrate through the surface to form a        damaged layer below that surface,    -   (iii) growing a layer of a wide-band gap material by chemical        vapour deposition on at least a portion of the surface of the        substrate through which ion implantation occurred, the material        of the grown layer having a characteristic different to that of        the region of the substrate adjacent the surface through which        ion implantation occurred, and    -   (iv) severing the substrate through the damaged layer.

The ion implantation should be carried out with ions which allow deeppenetration into the substrate, creating the damaged layer substantiallybelow the surface through which the ion implantation occurs. The ionssuitable to achieve this are typically ions of low atomic mass,preferably an atomic mass less than 21 and more preferably an atomicmass less than 13. Examples of suitable ions are helium and hydrogenions. The ions for the ion implantation are preferably of high energy,e.g. have an energy exceeding 5 keV. The precise depth of the damagedlayer can be accurately controlled by manipulating the energy and type(i.e. mass) of the implanted ions. Typically, the ion implantation dosewill exceed 1×10¹⁵ cm⁻².

Generally, the damaged layer will lie at a depth of 0.05 to 200 μm,typically 0.3 to 10 μm, below the surface through which ion implantationoccurred.

It is preferred that the region of the substrate between the surfacethrough which ion implantation occurs and the damaged layer issubstantially free of ion implantation doping damage.

The wide-band gap material may be silicon carbide, gallium nitride orthe like and is preferably diamond.

Generally, the layers will differ from each other in the characteristicwhich provides the layers with different electrical properties. Theproduct may comprise only two layers in contact with each other, or morethan two layers. When the product consists of more than two layers,adjacent layers, in contact with each other, will have differentcharacteristics. The interface between adjacent layers defines a sharpand well-defined interface between two regions having differentproperties. This is an important feature, particularly when the layeredproduct is to be used in an electronic application.

The surface through which the ion implantation occurs may be planar ornon-planar. Thus, the interface between adjacent layers may also beplanar or non-planar. When non-planar, the profile may be designed toprovide a specific useful feature for a device which includes thelayered product as a component.

The substrate may be natural or synthetic diamond, particularly CVDdiamond. The layer of grown wide-gap material may be CVD diamond ordoped CVD diamond.

In one particular form of the invention, the region of the substrateadjacent to the surface through which ion implantation occurred isuniformly doped. The dopant may be selected from nitrogen, boron,nickel, cobalt, iron, phosphorus, sulphur or other elements which canoccupy a lattice position, substitutional or otherwise, and provide theregion with useful properties, particularly electronic properties.

The substrate and layer of grown wide-gap material may have the samethickness or differ in thickness. Generally, the layers will differ inthickness.

The process of the invention minimises excessive complicated post growthprocessing and enables structures that contain thin layers of diamondwith properties very different to a second thicker layer to besynthesised. These structures have, for example, use in electronicapplications.

DESCRIPTION OF THE DRAWING

The drawing illustrates, as FIGS. 1(a) to 1(c), schematically the stepsin an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

The invention will now be illustrated with reference to the accompanyingdrawing. Referring to FIG. 1 a, a diamond substrate 10 has an uppersurface 12 and a lower surface 14. High energy ions are implanted in thediamond substrate 10 through surface 12, as illustrated by the arrows16. The ions will typically be of light atoms such as hydrogen ions. Theenergy of the hydrogen ions will typically be between 20 keV and 5 MeV.The dose will typically be between 1×10¹⁵ cm⁻² and 1×10²⁰ cm⁻². The ionspenetrate to a depth indicated by dotted line 18. The diamond region 22between the layer 18 and the surface 12 is not significantly damagedbecause the collision cross-section of the implanted ions is low athigher energies but rapidly increases as they slow down. Thus thesurface layer through which the ions are implanted suffers relativelylittle damage, with the majority of the damage being confined to thenarrow damage layer (region 18) required for subsequent release. Thedepth of region 18 below surface 12 may be in the range 0.05 μm to 200μm, and more typically in the range 0.3-10 μm.

The diamond substrate 10 may be natural, or synthesised by chemicalvapour deposition (CVD) or by high pressure high temperature (HPHT)techniques. This diamond will have characteristic electronic propertiesassociated with some specific incorporated defect. Selection of thisdiamond substrate from any source of diamond provides for the broadestpossible range of dopants, impurities or defects within the diamond withwhich to tailor its properties. The diamond substrate surface may beflat, for example a polished surface, or it may be curved or havenon-planar features such as trenches or raised features which maysubsequently form elements of, for example, an electronic devicestructure. This latter possibility arises because the nature of ionimplantation is to allow penetration of the ions down to a given depth,independent of the macroscopic variation in height of the substrate.This provides for device geometries that are not easily achieved indiamond by any other means. In one example of the invention, the dopantin the substrate may be present from growth of the diamond, e.g. nickel,cobalt or iron. As the dopant is present from growth of the substrate,the substrate diamond is free of the ion damage that would be associatedwith ion implantation doping, and the uniformity of the dopant is thatof the original synthesis technique not the very non-uniform dopingprofile associated with ion implantation.

An epitaxial diamond layer 20 of different properties is then grown byCVD on the surface 12 of the substrate 10 (FIG. 1 b). The conditionsnecessary to produce CVD diamond growth are well known in the art. Thethickness of the layer 20 will typically be greater than the region 22defined between the surface 12 and the damaged layer 18. This regionwill have a particular characteristic differing from that of the grownlayer 20. When the characteristic is imparted to the region by a dopant,that dopant will be uniformly distributed through the region. Thesurface 12 thus provides a very sharp boundary between the properties ofthe overgrown layer 20 and that of the region 22.

The diamond substrate is then severed along region 18, by immersing theproduct into an acid etch, annealing or using appropriateelectrochemical etching. The resulting product (FIG. 1 c) is a layeredproduct, in which diamond layer 20 has characteristics different to thatof diamond layer 22. Interface 24 provides a sharp boundary between thecharacteristics of the two layers.

Implantation damage in the released layer 22 is generally low, since iondamage is low until the ion energy is almost exhausted which occurs asit reaches the damage layer 18. However, when using a substrate with aplanar (preferably polished) surface, it is possible to reduce furtherthe effect of this ion damage by implanting to a greater depth than isrequired (say to 5 μm), and after release removing a portion of thethickness of the released layer 22 by polishing, to leave a thinnerfinal layer 22 (say 3 μm). This may be advantageous because the portionof diamond remaining had only higher energy ions traversing it, withproportionately lower ion damage, and the relatively heavily damagedregion adjacent to the damaged region 18 is then wholly removed.

The process can be repeated more than once. For example bi-layercomprising the thin top layer 22 on a thicker layer 20 formed accordingto the invention can be further implanted through surface 26 of layer 22into layer 20 to provide a damaged layer in layer 20. A thick CVDdiamond layer is grown on surface 26 of layer 22 and then the samplesevered along the implantation damaged layer. The result is a threelayer structure, comprising the thin layer 22 sandwiched between a thinportion of the layer 20 and the new CVD diamond layer.

EXAMPLE 1

A high purity diamond substrate produced using a CVD method known in theart with thickness 600 μm, is first implanted with 2 MeV oxygen ions toa dose of 1×10¹⁷ cm⁻². A thick (300 μm) boron doped single crystal CVDlayer which has, as measured by SIMS, 2×10¹⁹ B atoms/cm⁻³ is grown on asurface of this substrate. Following growth the layered product iselectrochemically etched to produce two samples: (i) a high puritydiamond layer that can be reused and (ii) a two layer product consistingof a 1 μm high purity diamond layer and a 300 μm boron doped diamondlayer in contact with the high purity diamond layer. This two layerproduct has an electronic application.

EXAMPLE 2

A boron doped (1×10¹⁹ cm⁻³) diamond substrate prepared using a CVDmethod with thickness 620 μm is first implanted with 2 MeV hydrogen ionsto a dose of 1×10¹⁹ cm⁻². A thick (300 μm) high purity single crystalCVD diamond layer is grown on to a surface of this substrate. Followinggrowth the layered product is electrochemically etched to produce twosamples: (i) a boron doped diamond plate which can be reused, and (ii) atwo layer product consisting of a 10 μm boron doped diamond layer and a300 μm high purity diamond layer. This two layer product has anelectronic application.

1. A process of making a product which comprises at least two layers incontact with each other, each layer being of a wide-band gap materialand each layer differing from the other layer in at least one property,including the steps of: (i) providing a substrate of a wide-band gapmaterial having a surface and a region adjacent the surface having aparticular characteristic, (ii) ion implanting the substrate through thesurface to form a damaged layer below that surface, (iii) growing alayer of a wide-band gap material by chemical vapour deposition on atleast a portion of the surface of the substrate through which ionimplantation occurred, the material of the grown layer having acharacteristic different to that of the region of the substrate adjacentthe surface through which ion implantation occurred, and (iv) severingthe substrate through the damaged layer.
 2. A process according to claim1 wherein the ions used in the ion implantation are ions of low atomicmass.
 3. A process according to claim 1 wherein the ions used in the ionimplantation have an atomic mass of less than
 21. 4. A process accordingto claim 1 wherein the ions used in the ion implantation have an atomicmass of less than
 13. 5. A process according to claim 1 wherein the ionsare helium or hydrogen ions.
 6. A process according to any one of thepreceding claims wherein ions of high energy are used in the ionimplantation.
 7. A process according to any one of the preceding claimswherein the ions used in the ion implantation have an energy exceeding 5keV.
 8. A process according to any one of the preceding claims whereinthe ion implantation dose exceeds 1×10¹⁵ cm⁻².
 9. A process according toany one of the preceding claims wherein severing of the substratethrough the damaged layer is achieved by acid etching, annealing orelectrochemical etching
 10. A process according to any one of thepreceding claims wherein the damaged layer lies at a depth of 0.05 to200 μm below the surface through which ion implantation occurred.
 11. Aprocess according to any one of the preceding claims wherein the damagedlayer lies at a depth of 0.3 to 10 μm below the surface through ionimplantation occurred.
 12. A process according to any one of thepreceding claims wherein the grown layer covers the entire surface ofthe substrate through which the ion implantation occurred.
 13. A processaccording to any one of the preceding claims wherein the layers differfrom each other in a characteristic which provides the layers withdifferent electrical properties.
 14. A process according to any one ofthe preceding claims wherein the wide band gap material is diamond. 15.A process according to any one of the preceding claims wherein thesubstrate is natural or synthetic diamond.
 16. A process according toany one of the preceding claims wherein the substrate is CVD diamond.17. A process according to any one of the preceding claims wherein thelayer of grown wide-gap material is boron-doped diamond.
 18. A processaccording to any one of the preceding claims wherein the region of thesubstrate adjacent the surface through which ion implantation occurredis uniformly doped.
 19. A process according to claim 18 wherein thedopant is selected from nitrogen, boron, nickel, cobalt, iron,phosphorus and sulphur.
 20. A process according to any one of thepreceding claims wherein the substrate and layer of grown wide-gapmaterial differ in thickness.
 21. A process according to any one of thepreceding claims wherein the surface through which ion implantationoccurs is planar.
 22. A process according to any one of the precedingclaims wherein the surface through which ion implantation occurs isnon-planar.
 23. A process according to claim 1 substantially as hereindescribed with reference to the accompanying drawing.
 24. A processaccording to claim 1 substantially as herein described in eitherExample.